Articular endoprostheses currently have a service life of several years, for example on average more than ten and up to fifteen years in the case of cemented hip endoprostheses. However, undesirable loosening of the articular endoprostheses can occur before the end of the usual service life. This can concern either septic or aseptic loosening. Aseptic loosening means that no microbial germs are detectable yet. There are many causes of aseptic loosening. Aseptic loosening is often related to abrasion at the sliding surfaces of articular endoprostheses. The loosening process in septic loosening is induced by microbial germs. This can either be early or late infections depending on the time of manifestation. Septic loosening is a very serious disease for the patient and associated with high additional costs. It is customary to perform a revision surgery in cases of aseptic and septic loosening alike. This can proceed as a one-stage or a two-stage revision surgery. Two-stage revision surgeries are very common in cases of septic loosening.
In a two-stage revision surgery, the infected articular endoprosthesis is removed in a first surgery (OP) followed by debridement and subsequent insertion of a temporary place-holder, a so-called spacer. Said spacer occupies for a number of weeks the space previously occupied by the revised endoprosthesis until the manifest infection has subsided. Said place-holder function is very important in order to effectively prevent muscular atrophy during this period of time and in order to stabilise the existing resection scenario. There are non-articulated and articulated spacers available. Articulated spacers or joint spacers replicate the function of the joint and allow the afflicted limbs to have a certain degree of mobility. This allows the patient to be mobilised early. Articulated spacers are current the state of the art. The spacer is removed in a second surgery, another debridement is done before implanting a cemented or cement-free revision articular endoprosthesis.
The use of spacers is originally based on the work of Hovelius and Josefsson (Hovelius L, Josefsson G (1979), “An alternative method for exchange operation of infected arthroplasty”, Acta Orthop. Scand. 50: 93-96). Other early work on spacers includes Younger (Younger A S, Duncan C P, Masri B A, McGraw R W (1997), “The outcome of two-stage arthroplasty using a custom-made interval spacer to treat the infected hip”, J. Arthroplasty 12: 615-623), Jones (Jones W A, Wroblewski B M (1989), “Salvage of failed total knee arthroplasty: the ‘beefburger’ procedure”, J. Bone Joint Surg. Br. 71: 856-857), and Cohen (Cohen J C, Hozack W J, Cuckler J M, Booth R E Jr (1988), “Two-stage reimplantation of septic total knee arthroplasty, Report of three cases using an antibiotic-PMMA spacer block”, J. Arthroplasty 3: 369-377). McPherson described a concept according to which spacers can be manufactured from bone cement exclusively (McPherson E J, Lewonowski K, Dorr L D (1995), “Techniques in arthroplasty. Use of an articulated PMMA spacer in the infected total knee arthroplasty”, J. Arthroplasty 10: 87-89).
Spacers equipped with antibiotics for temporary replacement of knee, hip, and shoulder endoprostheses are available on the market. It is disadvantageous though that the antibiotics contained therein are pre-determined and cannot be adapted specifically to suit the antibiogram of the microbial germs found to be present.
Spacers are often shaped from cement dough by the physician in the course of a surgery or are cast by the physician using elastic silicone moulds. Alternatively, industrially produced spacers have been commercially available in a range of standard sizes for a number of years. Said pre-made spacers can be implanted directly by the physician without any major preparatory work, whereby the spacers are usually fixed to the bone tissue with polymethylmethacrylate bone cement. The advantage of industrially produced spacers is that the surface quality of the sliding surfaces of the pre-made spacers is generally markedly better as compared to the spacers produced intra-operatively. The sliding surfaces are the surfaces of articulated spacer parts that slide on each other or roll over each other in order to replicate the function of a joint by the spacer. Moreover, using pre-made spacers can save valuable surgery time and simplifies the entire surgical process markedly since the time-consuming and laborious step of spacer production is omitted.
Casting spacers with polymethylmethacrylate bone cement using elastic casting moulds, such as, for example, silicone moulds, leads to spacers having a relatively good surface quality. However, it is an inherent problem of industrial production that casting moulds made of rubber-elastic material are sensitive to wear and tear and are therefore not suitable for the production of spacers in large quantities. Moreover, the production of silicone moulds is expensive. It is therefore reasonable to use casting moulds that are largely resistant to wear and tear.
Our own casting experiments using wear-resistant casting moulds made of metal or non-rubber-elastic plastic materials and polymethylmethacrylate bone cement dough at room temperature showed that irregular pore-like depressions are formed on the surface of the spacers. These pores and/or surface defects are particularly bothersome in the region of the sliding surfaces. Pores or other irregularities on the sliding surfaces can lead to increased abrasion while the spacer is implanted in the patient, which might later favour or trigger inflammatory processes if the abraded material is not removed during explanation of the spacer. Moreover, pores and other surface defects can act as fissure initiation points and thus adversely affect the mechanical stability of the spacers. It is feasible to reduce or eliminate pores and other surface defects by grinding and polishing. This type of reworking is time-consuming and expensive, though.
Accordingly, it is the object of the invention to overcome the disadvantages of the prior art. In particular, a method and a device for producing spacers from bone cement, in particular polymethylmethacrylate bone cement, are to be developed that can be used with wear-resistant, dimensionally stable casting moulds to produce spacers on an industrial scale, whereby the formation of pores or other surface defects is largely excluded.